1. Field of the Invention
The present invention relates to a lighting apparatus, and more particularly, to a lighting apparatus and projection display apparatus using the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for minimizing etendue.
2. Description of the Related Art
Generally, a lighting apparatus can be used for a projection engine for example. An optical system of a projection engine is mainly divided into an illumination system and a projection system. A major performance of the illumination system is how efficiently the light emitted from a light source can be condensed to an imager.
If a light source is a point light source, it is theoretically able to collect all light emitted from a light source. Yet, the point light source substantially does not exist. And, all kinds of light sources have their own surfaces or volumes, respectively. Hence, theoretical limitation is put on light-condensing efficiency. Specifically, a light source used for a projection engine is an ultra high-pressure discharge lamp such as an arc lamp. Light is emitted from the arc lamp using a discharge occurring between electrodes. And, such a light source as the arc lamp is not the point light source but a light source having a length of an arc gap.
As a light source gets closer to a point light source (i.e., as an arc gap gets smaller), light-condensing efficiency becomes higher. Yet, since the arc gap is associated with an expected life span of a lamp, it is difficult to reduce the arc gap. Currently, an arc gap provided by an advanced lamp manufacturer is about 1 mmm.
Thus, since a light source has constant volume or length, theoretical light-condensing efficiency is limited by etendue. And, the etendue is defined by Formula 1.∫∫cos θdAdΩ  [Formula 1]
In this case, ‘dA’ indicates a unit area of a light source, ‘dΩ’ indicates a unit solid angle of a light source, and ‘θ’ indicates an angle between a normal direction and a solid angle center of a light source.
As can be seen from Formula 1, etendue is a value associated with an area and a solid angle. A light source having a small etendue, i.e., a light source closer to a point light source can condense light to have a small area and a small solid angle using a lens or mirror. In this case, the etendue can be defined by Formula 2.E=πA sin2(θ1/2)=πA/4(F/#)2  [Formula 2]
In this case, ‘A’ indicates an area of a light source or imager, ‘θ1/2’ indicates ½ of an angle at which the outermost of an entrance pupil of projection lens is shown at the imager and ‘F/#’ indicates a rate of a focus distance over a diameter of the entrance pupil.
In the side of manufacture cost, a projection engine becomes advantageous in having a smaller imager and a greater F/#. Hence, an illumination system should illuminate a small area with a big F/#, i.e., a smaller solid angle. Yet, since etendue of a light source is limited, it is unable to condense light on a small area, which is demanded by a projection system, by a small solid angle. Hence, it is unable to reduce etendue even by any kind of optical systems employing Lagrange Invariant in case that a light source has a wide area and that a projected light has a big angle for radiation. This is the limitation of the light-condensing efficiency restricted by the etendue of light source. And, a real system cannot overcome the theoretical efficiency.
Moreover, etendue of a light-receiving part that is illuminated can be calculated. In a projection engine, etendue of a MD (micro display) panel can be calculated. Namely, the etendue of the light-receiving part is determined by an area of the light-receiving part and F/# that is used. Yet, if etendue of a light source is greater than that of the light-receiving part, the light-receiving part is unable to accept rays amounting to the corresponding etendue difference. Thus, the etendue of the light source and the etendue of the light-receiving part put limitation on efficiency of a lighting apparatus. Hence, illumination designers seriously take the etendue into consideration in designing lighting apparatuses.
Meanwhile, to solve the aforesaid problems caused by the difficulty in reducing the arc lamp size associated with the expected life span of the lamp used as a light source, an LED (light emitting diode) is studied to use for a light source of a projection engine (U.S. Pat. No. 6,224,216). The LED has an expected life-span longer than that of the arc lamp, a reduced size, a lightened weight, a fast response speed, a low-voltage drive, a wide color-expression range, etc.
Since the LED fails in providing a sufficient light output as a single device so far, several LEDs are needed to secure the necessary light output.
However, the LED has its etendue as well. So, in case of using several LEDs to secure the light output, the etendue of the entire light sources is raised.